1. Field of the Invention
The present invention relates to a cooling drum for continuous-casting machines for producing thin metallic strip, and especially suitable for thin drum type continuous-casting machines.
2. Description of the Related Art
Currently, in the continuous casting of metals, desirably a thin strip with a shape near to that of the final product is provided, to reduce the production cost and to produce a new material. To this end, many methods have been proposed, several of which have been practiced in manufacture, but none of these methods can provide the necessary productivity and thin strip quality.
These continuous-casting methods for manufacturing thin strips include those which use a relatively simple machine structure, such as of a twin drum type using a pair of drums provided with an interior cooling system, a single drum type using a cooling drum, and a drum-belt type in which a molten metal pool is formed between a drum and a belt, etc. In these continuous-casting methods, it is important to stably provide a strip having a high quality surface, since they have been developed to produce a thin strip which can minimize the reduction rate in later rolling steps, in contrast with slabs produced by the ordinary continuous-casting machine and to be hot rolled at a high reduction ratio. Surface defects such as thickness fluctuation, if present on a thin strip, will cause surface defects on a final product and may cause an extreme impairment of the product value.
Many methods have been studied of stably obtaining a good surface quality of a cast strip.
U.S. Pat. No. 3,345,738 (issued Oct. 10, 1967) to Mizikar et al. discloses a method of producing steel strip of a uniform thickness by direct casting, in which a chill surface is brought into contact with molten steel so that a thin skin of steel solidifies thereon to create in that skin a surface pattern of distributed point indentations. To this end, a chill surface is provided with knurls formed thereon, for example, by cutting a group of parallel V-grooves and crossing them with another group of parallel V-grooves. This knurled chill surface has, however, the following essential drawbacks. The knurls are defined by the continued grooved portions along which air gaps may be continuously formed between the chill surface and the solidified skin to form the continued skin portions having a delayed solidification which will cause surface defects such as cracks. Moreover, although the solidified shell will be unified, indentations are formed on the steel strip surface and may be retained as a surface defects even after rolling.
Japanese Unexamined Patent Publication (Kokai) No. 60-184449 proposed a cooling drum having a circumferential surface provided with dimples to form air gaps as a heat insulating layer between the cooling drum and a solidified shell. The air gap lowers the heat extraction capacity of the cooling drum and the molten metal is cooled in a milder cooling condition, i.e., is more slowly cooled. This has been expected to give the solidified shell a uniform thickness over the strand width and to enable the production of a thin strip having a good shape characteristic.
The present inventors have experimentally found, however, that the expected effect can not be obtained even if uniformly disposed dimples having a predetermined depth are provided on the circumferential surface of a cooling drum and are maintained in the initial state. For example, large or continuously disposed dimples on the circumferential surface of a drum cause the formation of unevenness on the surface of a resulting thin strip, and this unevenness promotes the concentration of thermal stress which leads to surface cracking. Dimples having a linear or angular shaped opening portion provided on the circumferential surface of a drum also cause an uneven surface of a thin strip, with resulting numerous cracks, since the solidified shell is mechanically sensitive to the corners of such shaped dimples.